Sub-cycle nanotip field emission of electrons driven by air plasma generated THz pulses

Terahertz pulses generated by two-color laser plasmas have reported peak field strengths exceeding MV/cm, and when illuminating metal nanotips, the near-field enhancement at the tip apex should result in extremely high bunch charges and electron energies via sub-cycle cold-field emission. Here, electron emission from tungsten nanotips driven by THz pulses generated by a long filament air-plasma is reported. Electron energies up to 1.1 keV and bunch charges up to 2× 10⁵ electrons per pulse were detected, well below values expected for peak field calculated via the time-averaged Poynting vector. Investigations revealed a failure in the use of the time-averaged Poynting vector when applied to long filament THz pulses, due to spatiotemporal restructuring of the THz pulse in the focus. Accounting for this restructuring significantly reduces the field strength to approximately 160 kV/cm, consistent with the observed electron bunch charges, peak energies, and their dependence on the tip position in the THz focus. Despite these findings, our results surpass previous THz plasma-driven electron generation by an order of magnitude in both electron energy and bunch charge, and a path to increasing these by an additional order of magnitude by modification of the THz optics is proposed.